The present invention relates to pneumatic and hydraulic cylinders.
Hydraulic and pneumatic cylinders suffer from the disadvantage that should there be any leakage, the cylinder will either extend or retract depending on its preloaded condition.
It is the object of the present invention to overcome or substantially ameliorate the above disadvantage.
There is disclosed herein a hydraulic or pneumatic cylinder including:
a bore, the bore generally surrounding a chamber having opposite ends;
a piston in said bore and co-operating therewith to divide said chamber into a first and a second sub-chamber;
a piston rod fixed to and extending from said piston and extending beyond one of said ends;
means closing said opposite ends;
releasable locking means to retain said piston at a desired location within said bore, said locking means being released to permit movement of said piston upon delivery of a fluid under pressure to one of said chambers; and
duct means to provide for fluid flow from said second sub-chamber to said first sub-chamber when said locking means is engaged with said piston to retain said piston at said desired location.
Preferably, the locking means includes a pawl mounted in said body, and a recess in said piston, said recess being alignable with said pawl so as to be engaged therewith to retain the piston at said desired location, said pawl being movable between a first position at which it is engaged within said recess and a second position spaced from said recess to provide for movement of said piston.
Preferably, the cylinder includes a spring urging the pawl into engagement with said recess.
Preferably, the delivery of fluid under pressure to said first sub-chamber causes movement of said pawl from the first position to the second position thereof.